When did they start diamond mining?

Although alluvial diamonds had been found in India as early as 6,000 BC, and in Brazil beginning in the 1700s, the first mine opened in the 1870s, after according to Wikipedia: "... the discovery of the Diamond Fields in South Africa."

(Again, from Wikipedia:

"Alluvium -- from the Latin, alluvius, from alluere, 'to wash against' -- is loose, unconsolidated -- not cemented together into a solid rock -- soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting.")